Proteins are used in a wide range of applications in the fields of pharmaceuticals, veterinary products, cosmetics and other consumer products, foods, feeds, diagnostics, industrial chemistry and decontamination. At times, such uses have been limited by constraints inherent in proteins themselves or imposed by the environment or media in which they are used. Such constraints may result in poor stability of the proteins, variability of performance or high cost.
It is imperative that the higher order three-dimensional architecture or tertiary structure of a protein be preserved until such time that the individual protein molecules are required to perform their unique function. To date, a limiting factor for use of proteins, particularly in therapeutic regimens, remains the sensitivity of protein structure to chemical and physical denaturation encountered during delivery.
Various approaches have been employed to overcome these barriers. However, these approaches often incur either loss of protein activity or the additional expense of protein stabilizing carriers or formulations.
One approach to overcoming barriers to the widespread use of proteins is crosslinked enzyme crystal (“CLEC™”) technology [N. L. St. Clair and M. A. Navia, J. Am. Chem. Soc., 114, pp. 4314-16 (1992)]. See also PCT patent application PCT/US91/05415. Crosslinked enzyme crystals retain their activity in environments that are normally incompatible with enzyme function. Such environments include prolonged exposure to proteases, organic solvents, high temperature or extremes of pH. In such environments, crosslinked enzyme crystals remain insoluble, stable and active.
Despite recent progress in protein technology generally, two problems which are discussed below continue to limit the use of biological macromolecules in industry and medicine. The first problem relates to molecular stability and sensitivity of higher order tertiary structures to chemical and physical denaturation during manufacturing and storage. Second, the field of biological delivery of therapeutic proteins requires that vehicles be provided which release native proteins, such as proteins, glycoproteins, enzymes, antibodies, hormones, nucleic acids and peptides at a rate that is consistant with the needs of the particular patient or the disease process.
Macromolecule Stability
Numerous factors differentiate biological macromolecules from conventional chemical entities, such as for example, their size, conformation and amphiphilic nature. Macromolecules are not only susceptible to chemical, but also physical degradation. They are sensitive to a variety of environmental factors, such as temperature, oxidizing agents, pH, freezing, shaking and shear stress [Cholewinski, M., Luckel, B. and Horn, H., Acta Helv., 71, 405 (1996)]. In considering a macromolecule for drug development, stability factors must be considered when choosing a production process.
Maintenance of biological activity during the development and manufacture of pharmaceutical products depends on the inherent stability of the macromolecule, as well as the stabilization techniques employed. A range of protein stabilization techniques exist; including:
a) Addition of chemical “stabilizers” to the aqueous solution or suspension of protein. For example, U.S. Pat. No. 4,297,344 discloses stabilization of coagulation factors II and VIII, antithrombin III and plasminogen against heat by adding selected amino acids. U.S. Pat. No. 4,783,441 discloses a method for stabilizing proteins by adding surface-active substances. U.S. Pat. No. 4,812,557 discloses a method for stabilizing interleukin-2 using human serum albumin. The drawback of such methods is that each formulation is specific to the protein of interest and requires significant development efforts.
b) Freeze/thaw methods in which the preparation is mixed with a cryoprotectant and stored at very low temperatures. However, not all proteins will survive a freeze/thaw cycle.
c) Cold storage with cryoprotectant additive, normally glycerol.
d) Storage in the glass form, as described in U.S. Pat. No. 5,098,893. In this case, proteins are dissolved in water-soluble or water-swellable substances which are in amorphous or glassy state.
e) The most widely used method for the stabilization of proteins is freeze-drying or lyophilization [Carpenter, J. F., Pical, M. J., Chang, B. S. and Randolph, T. W., Pharm. Res., 14:(8) 969 (1997)]. Whenever sufficient protein stability cannot be achieved in aqueous solution, lyophilization provides the most viable alternative. One disadvantage of lyophilization is that it requires sophisticated processing, is time consuming and expensive [Carpenter, J. F., Pical, M. J., Chang, B. S. and Randolph, T. W., Pharm. Res., 14: (8) 969 (1997) and literature cited therein]. In addition, if lyophilization is not carried out carefully, most preparations are at least partially denatured by the freezing and dehydration steps of the technique. The result is frequently irreversible aggregation of a portion of protein molecules, rendering a formulation unacceptable for parenteral administration.
The vast majority of protein formulations produced by the above-described techniques require cold storage, sometimes as low as −20° C. Exposure to elevated temperatures during shipping or storage can result in significant activity losses. Thus, storage at elevated, or even ambient temperatures, is not possible for many proteins.
Proteins, peptides and nucleic acids are increasingly employed in the pharmaceutical, diagnostic, food, cosmetic, detergent and research industries. There is a great need for alternative stabilization procedures, which are fast, inexpensive and applicable to a broad range of biological macromolecules. In particular, stabilization procedures are needed that do not rely on the excessive use of excipients, which can interfere with the functions of those biological macromolecules.
The stability of small molecule crystalline drugs is such that they can withstand extreme forces during the formulation process (see U.S. Pat. No. 5,510,118). Forces associated with milling nanoparticles of crystalline material of relatively insoluble drugs include: shear stress, turbulent flow, high impact collisions, cavitation and grinding. Small molecular crystalline compounds have been recognized as being much more stable toward chemical degradation than the corresponding amorphous solid [Pical, M. J., Lukes, A. L., Lang, J. E. and Gaines, J. Pharm. Sci., 67, 767 (1978)]. Unfortunately, crystals of macromolecules, such as proteins and nucleic acids, present additional problems and difficulties not associated with small molecules.
For most of this century, science and medicine have tried to solve the problem of providing insulin in a useful form to diabetics. Attempts have been made to solve some of the problems of stability and biological delivery of that protein. For example, U.S. Pat. No. 5,506,203 describes the use of amorphous insulin combined with an absorbtion enhancer. The solid state insulin was exclusively amorphous material, as shown by a polarized light microscope.
Jensen et al. co-precipitated insulin with an absorbtion enhancer for use in respiratory tract delivery of insulin (See PCT patent application WO 98/42368). Here, the absorbtion enhancer was desribed as a surfactant, such as a salt of a fatty acid or a bile salt. Insulin crystals of less than 10 micrometers in diameter and lacking zinc were produced by S. Havelund (See PCT patent application WO 98/42749). Similarly, crystals were also produced in the presence of surfactants to enhance pulmonary administration.
To date, those of skill in the art recognize that the greatly enhanced stability of the crystalline state observed for small molecules does not translate to biological macromolecules [Pical, M. J. and Rigsbee, D. R., Pharm. Res., 14:1379 (1997)]. For example, aqueous suspensions of crystalline insulin are only slightly more stable (to the degree of a factor of two) than corresponding suspensions of amorphous phase [Brange, J., Langkjaer, L., Havelund, S. and Volund, A., Pharm. Res., 9:715 (1992)]. In the solid state, lyophilized amorphous insulin is far more stable than lyophilized crystalline insulin under all conditions investigated [Pical, M. J. and Rigsbee, D. R., Pharm. Res., 14:1379 (1997)].
Until now, formulations of crystalline proteins have been available only for very small proteins, e.g. proteins with molecular weights of less than 10,000 Daltons. Molecular weight has profound effect on all properties of macromolecules, including their macromolecular volume, hydration, viscosity, diffusion, mobility and stability. [Cantor, C. R and Schimmel, P. R, Biophysical Chemistry, W. H. Freeman and Co., New York, 1980].